Re: [Qemu-devel] [RFC] vhost-user: introduce F_NEED_ALL_IOTLB protocol feature

[Jason Wang]

On 2018年04月12日 09:44, Tiwei Bie wrote:
> On Wed, Apr 11, 2018 at 08:37:17PM +0300, Michael S. Tsirkin wrote:
> > On Wed, Apr 11, 2018 at 04:38:53PM +0800, Tiwei Bie wrote:
> > > On Wed, Apr 11, 2018 at 04:01:19PM +0800, Jason Wang wrote:
> > > > On 2018年04月11日 15:20, Tiwei Bie wrote:
> > > > > This patch introduces VHOST_USER_PROTOCOL_F_NEED_ALL_IOTLB
> > > > > feature for vhost-user. By default, vhost-user backend needs
> > > > > to query the IOTLBs from QEMU after meeting unknown IOVAs.
> > > > > With this protocol feature negotiated, QEMU will provide all
> > > > > the IOTLBs to vhost-user backend without waiting for the
> > > > > queries from backend. This is helpful when using a hardware
> > > > > accelerator which is not able to handle unknown IOVAs at the
> > > > > vhost-user backend.
> > > > >
> > > > > Signed-off-by: Tiwei Bie<address@hidden>
> > > > > ---
> > > > > The idea of this patch is to let QEMU push all the IOTLBs
> > > > > to vhost-user backend without waiting for the queries from
> > > > > the backend. Because hardware accelerator at the vhost-user
> > > > > backend may not be able to handle unknown IOVAs.
> > > > >
> > > > > This is just a RFC for now. It seems that, it doesn't work
> > > > > as expected when guest is using kernel driver (To handle
> > > > > this case, it seems that some RAM regions' events also need
> > > > > to be listened). Any comments would be appreciated! Thanks!
> > > > Interesting, a quick question is why this is needed? Can we just use exist
> > > > IOTLB update message?
> > > Yeah, we are still using the existing IOTLB update messages
> > > to send the IOTLB messages to backend. The only difference
> > > is that, QEMU won't wait for the queries before sending the
> > > IOTLB update messages.
> > So I have a concern with that, in that without any flow
> > control the socket buffer used by vhost-user might become
> > full.
> Each IOTLB update message needs a reply. So I think it
> won't happen.

Is this what we've already done now? I don't find any statement on this 
in vhost-user.txt?

"""
 * VHOST_USER_IOTLB_MSG

      Id: 22
      Equivalent ioctl: N/A (equivalent to VHOST_IOTLB_MSG message type)
      Master payload: struct vhost_iotlb_msg
      Slave payload: u64

      Send IOTLB messages with struct vhost_iotlb_msg as payload.
      Master sends such requests to update and invalidate entries in 
the device
      IOTLB. The slave has to acknowledge the request with sending zero 
as u64
      payload for success, non-zero otherwise.
      This request should be send only when VIRTIO_F_IOMMU_PLATFORM feature
      has been successfully negotiated.
"""

And you probably need to modify the following statement:

"""
The master isn't expected to take the initiative to send IOTLB update 
messages,
as the slave sends IOTLB miss messages for the guest virtual memory areas it
needs to access.
"""

Thanks


> Best regards,
> Tiwei Bie